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Trusses, beams and frames

  1. Dec 17, 2011 #1
    What makes the three mechanical objects Truss members, beams, and frames having different characteristics related to force systems. Example truss members only exhibit tension and compressional forces, that's different from others.
  2. jcsd
  3. Dec 18, 2011 #2


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    The difference is in the forces they "exhibit", as you stated. In beam elements, in general, you have a longitudinal force (tension or compression), a bending moment and a shear force. Of course, this goes for a 2D beam element. A frame is a system composed of beam elements, while a truss is a system composed of truss elements.
  4. Dec 20, 2011 #3
    Thank you!! I have got it...
    But still what i wonder is... What's the physical feature that makes the beams different from the truss members. Seriously initially i expected them to be used interchangeably....
  5. Dec 20, 2011 #4


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    Godwin Kessy: A truss member has a ball joint (or pin) on each end, and has no internal transverse applied load (i.e., no transverse load applied between its ends). Beam members and frame members do not have these limitations.
  6. Dec 20, 2011 #5
    Neither reply is incorrect but neither is complete.

    Firstly you need to distinguish between applied forces (loads) and resisting forces. Remember that support reactions are considered as applied forces.

    Secondly you need to distinguish between the structure as a whole and the individual members.

    The same loads can be applied to a truss, a frame or a beam considered as a whole structure. The difference lies in the response of the members ie the resisting forces.

    So a truss (as a structure) supported on two knife edges is subject to the same shear forces and bending moments as a beam carrying the same loads and supported on the same knife edges.

    Trusses are designed as an assemblage of relatively (to the whole structure) slender members that offer only axial resistance forces ie tension and compression. As previously stated their joints are considered as free to rotate it pins, hinges or whatever. Further forces applied to the structure as a whole (loads) are applied only at these joints.
    The upshot of this is that no bending could be transmitted via a joint from one member to another.
    Another consequence is that the reactions for a truss are necesarily forces, not moments.

    Beams on the other hand offer resisting shear forces and moments directly and so can readily carry applied loads at any point along the beam.
    The reactions for a beam may be forces, moments or both.

    Frames contain members that are linked by joints that can transmit a moment from one member to the next. These members offer resisting moments and are thus like beams, as previously noted.
    Not all the joints need to be able to transmit moment and not all the moment may be transmitted.
    Consequently frames may have reactions that are simple forces, moments or both.

    Another consequence of all this is that the internal resisting forces of truss members will be either tension or compression, but not both.
    On the other hand, the internal resisting forces in beams always include both tension and compression beams always

    Hope this helps.
    Last edited: Dec 20, 2011
  7. Dec 21, 2011 #6
    Thanks all!! It was perfect!!
    In Studiot's explanation I happen to got another question, When solving the moment and shear diagrams for loaded beams with two supports I realized that when solving the system from the left support the assumed directions of the shear force and moments at distance x would give a different shear and moment diagrams... Have any one seen that!!? And is the suggestion gone be that the use of the standard assumption should be put in consideration.?
  8. Dec 21, 2011 #7
    Sorry, Godwin I didn't quite understand your last question.
    Was it about sign conventions?
  9. Dec 23, 2011 #8
    You know what we normally assume direction to solve for shear force and moment at distance x. Now I have observed that when we start from the left support the shear is assumed downward direction and moment anticlockwise. And the reverse form the other end. Now is that the standard assumption or it has some logic with it... I don't just get it. Since if you try to reverse the assumption you get the same graphs but in the opposite (meaning negative). So the graphs differ from the one solved in maybe the text book. Got it?
    Last edited: Dec 23, 2011
  10. Jan 11, 2012 #9
    Studiot I Have your reply already!!
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